In the United States about 40,000 corneal transplant operations are performed each year. While success of such surgery may depend upon a number of other factors, one factor that always has an effect on the outcome is the condition of the donor cornea. In the United States, a donor cornea must be transplanted within 7 days of harvesting. Outside the United States donor corneas may be used up to 14 days after harvesting. Additionally, it is essential to use only corneas which have not been modified (e.g., the subject of photorefractive surgery).
The growth of refractive surgery over the last five years has been dramatic. In the August 2000 issue of Archives of Ophthalmology, P. J. McDonnell, M.D. states that this year alone over 1,500,000 refractive procedures will be performed. As beneficial as these procedures are, the individual corneas are permanently altered, which makes them unsuitable for corneal transplanting.
The increase in refraction surgery increases the likelihood that a modified cornea will be harvested for transplant purposes. Unfortunately, it is generally not possible to conclusively tell, either visually or under a microscope, whether such a donor cornea has been subjected to a surgical procedure or otherwise altered.
Even when properly stored in a container (e.g., a Chiron Ophtholmics cornea container) filled with Optisol® or another appropriate solution, a donated cornea changes optically in the 14 day time period referenced above. The interior starts to develop optical scatter sources and the optical power of the cornea changes. The scatter resources manifest themselves as randomly distributed optical aberrations which increase over time. It is believed that this is caused by the cells of the harvested cornea not being able to reject waste material. The change in optical power is believed to be caused by an overall relaxation of the tissue. Regardless of the cause, the net result is that these aberrations produce scintillation and static aberrations when a beam of light is passed through a donated cornea.
PCT/GB99/00658 (International Publication No. WO 99/467768), based on applications filed in Great Britain on Mar. 10, 1998 and Dec. 23, 1998, discloses a three dimensional imaging system including a lens and a distorted diffraction grating which images objects located at different distances from the grating simultaneously and spatially separated in a single image plane. The grating is distorted according to a quadratic function so as to cause the images to be formed under different focus conditions. It is stated that the system is useful for simultaneously imaging multiple layers within a three dimensional object field, and has applicability in a number of fields including optical information storage, imaging short-time scale phenomena, microscopy, imaging three dimensional object structures, passive ranging, laser beam profiling, wavefront analysis, and millimeter wave optics. The ability to make wavefront measurements is not disclosed or claimed.
P. M. Blanchard et al., “Multi-Plane Imaging With a Distorted Grating,” Proceedings of the 2nd International Workshop on Adaptive Optics for Industry and Medicine, World Scientific, pp. 296-301, Jul. 12-16, 1999, describe a technique for simultaneously imaging multiple layers within an object field onto the detector plane of a single detector. The authors, who are the named inventors in PCT/GB99/00658, state that the imaging of multiple layers within an object field is “useful in many applications including microscopy, medical imaging and data storage.” (See page 296.) The apparatus includes the use of a binary diffraction grating in which the lines are distorted such at each different level of defocus is associated with each diffraction order. When such a grating is placed in close proximity to a lens, the grating creates multiple foci of the image. This multi-foci effect enables the imaging of multiple object planes onto a single image plane.
L. J. Otten et al. “3-D Cataract Imaging System,” Proceedings of the 2nd International Workshop in Adaptive Optics for Industry and Medicine, World Scientific, pp. 51-56, describe optics and an associated diagnostic system for volumetric, in vivo imaging of the human lens to characterize or grade cataracts. The described method and apparatus are based on the use of a distorted grating (of the type disclosed in PCT/GB99/00658 and the Blanchard et al. paper, supra) in conjunction with a focusing lens and a re-imaging lens. (See FIG. 1 of this reference.) The quadratic phase shift, introduced by the grating, leads to a different degree of defocus in all diffraction orders, which produces a series of images of different layers of the cataract, each with different defocus conditions, simultaneously and side-by-side on the detector. Thus, in-focus images of different object planes are produced.
Analysis of the optical images referenced above requires the use of the Intensity Transport Equation (I.T.E.) and the employment of a Green's function to produce a wavefront map. S. Woods, P. M. Blanchard and A. H. Greenaway, “Laser Wavefront Sensing Using the Intensity Transport Equation,” Proceedings of the 2nd International Workshop on Adaptive Optics for Industry and Medicine, World Scientific, pp. 260-265, Jul. 12-16, 1999, describe both the I.T.E. and a Green's function solution thereto in conjunction with laser wavefront sensing.